1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device which suppresses an increase in driving voltage and which exhibits improved emission performance.
2. Background Art
Hitherto, various known Group III nitride semiconductor light-emitting devices are known, and some of them are disclosed in Japanese Patent Application Laid-Open (kokai) Nos. H11-191639 and 2007-180499. Japanese Patent Application Laid-Open (kokai) No. H11-191639 discloses a Group III nitride semiconductor light-emitting device having a structure in which a light-emitting layer is stacked on n-type layers; i.e., a third layer bonded to the light-emitting layer, a second layer bonded to the third layer, and a first layer bonded to the second layer. The second layer has a superlattice structure formed of two layers (AlGaN layer and GaN layer) or a superlattice structure formed of two layers (AlGaN layer and InGaN layer), and the third layer has an impurity concentration lower than that of the second layer. This configuration of the device reduces driving voltage.
Japanese Patent Application Laid-Open (kokai) No. 2007-180499 discloses a Group III nitride semiconductor light-emitting device including a substrate, an n-type semiconductor layer on which an n-electrode is formed, and an intermediate layer provided between the substrate and the n-type semiconductor layer and having a periodic structure formed of AlGaN, GaN, and InGaN. The intermediate layer improves the crystallinity of the n-type semiconductor layer which serves as a current path and on which the n-electrode is formed, thereby improving the reliability of the device. In this device, propagation of threading dislocations generated in a layer provided below the intermediate layer into a layer provided above the intermediate layer is blocked by means of the periodic structure of the intermediate layer. This configuration of the device improves the crystallinity of the layer provided above the intermediate layer, thereby improving emission performance, reverse breakdown voltage, and electrostatic breakdown voltage.
However, as described in Japanese Patent Application Laid-Open (kokai) No. H11-191639, when the second layer, which is not directly bonded to the light-emitting layer, has a superlattice structure formed of AlGaN and GaN layers or a superlattice structure formed of AlGaN and InGaN layers, reduction of driving voltage fails to be attained. In addition, since the AlGaN layer acts as a barrier against electrons, resistance to electrons is not reduced, and confinement of holes in the light-emitting layer is not improved. In the device disclosed in Japanese Patent Application Laid-Open (kokai) No. 2007-180499, the light-emitting layer exhibits improved crystallinity, but confinement of holes in the light-emitting layer fails to be attained, and resistance to electrons is not reduced. Thus, these conventional devices fail to achieve both reduction of driving voltage and improvement of emission performance.